Processes for the continuous production of melamine from urea are known. An example is the so-called BASF low-pressure process, as it is described in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, Chap. 4.1.1., 1998 Electronic Release.
The process according to the prior art proceeds at high temperatures in the gas phase. The starting material, the urea, is charged into a fluidized-bed reactor in liquid form, is fluidized therein by a NH3—CO2 process gas mixture, evaporated at temperatures of 390 to 410° C., converted into melamine in the presence of an aluminum catalyst via the intermediate product isocyanic acid, wherein as further reaction products ammonia and carbon dioxide and as by-products melem and melam are obtained. After leaving the fluidized-bed reactor, this gas mixture first is cooled to about 340° C. in a gas cooler, in order to crystallize out the by-products melem and melam, which are separated from the gas stream in a subsequently traversed gas filter, together with catalyst particles entrained from the fluidized-bed reactor. Subsequently, the gas is guided into a crystallizer in which it is cooled to 190 to 220° C. for crystallizing out the melamine. The mixture of the residual gaseous constituents, ammonia, carbon dioxide and isocyanic acid and crystallized powdery melamine, is passed from the crystallizer into a separator in which the melamine is separated from the gas and discharged as process product. By means of a blower, the gaseous constituents are passed from the separator into a scrubber in which the gas is washed with liquid urea, wherein the isocyanic acid contained in the gas and other by-products of the reactions taking place in the fluidized-bed reactor are washed out from the gas, move into the liquid urea and hence remain in the process. From the urea circuit of the scrubber a partial stream is branched off and, mixed with ammonia, fed into the fluidized-bed reactor as starting material for the melamine production. The gas mixture of ammonia and carbon dioxide, which in the scrubber is liberated from isocyanic acid residues, in part is used as fluidizing gas in the fluidized-bed reactor and in part is fed into the crystallizer as cooling gas for crystallizing out the melamine.
In the scrubber, the process gas is cooled down to 135 to 143° C., as a low temperature which lies as close as possible above the melting temperature of the urea of 130 to 135° C., promotes a substantial conversion of the isocyanic acid into urea. In addition, a rather low temperature is advantageous for the use as cooling gas in the crystallizer.
It is characteristic for this BASF low-pressure process that the entire amount of process gas, after separating the melamine, is guided over the scrubber operated with liquid urea, wherein the gaseous and solid product and by-product residues move into the urea and, by being fed into the fluidized-bed reactor together with the same, remain in the process circuit and are not discharged from the process with the excess gas and get lost. In the scrubber, the product and by-product residues for the most part are again converted into urea, so that it is avoided that these substances repeatedly pass through the hot fluidized-bed reactor and thereby form chemical compounds which might contaminate the melamine.
A disadvantage of this treatment of the entire process gas in the scrubber consists in that a process gas saturated with urea thereby also is supplied to the conveying means of the fluidized-bed reactor, so that disturbing urea deposits repeatedly are formed at its inlet.
In another process for producing melamine, which is set forth in the Chinese laid-open specification CN 1188761A, Jiang Dazhou et al., this disadvantage, i.e. the tendency to form urea deposits, has been avoided by completely passing the process gas used as fluidizing gas past the urea scrubber. In this process, the process gas, after leaving the urea scrubber, is completely passed through the melamine crystallizer as cooling gas, is heated up thereby and is supplied to the conveying means of the fluidized-bed reactor with the temperature reached thereby.
In this process it is disadvantageous that the temperature of the process gas necessarily corresponds to the gas temperature existing in the crystallizer and cannot, independent thereof, be adjusted to the temperature optimally suited for the condenser or the blower.
In principle, setting an upper limit for the gas temperature is very important for the manufacturing costs and for the operational safety of a condenser or blower. A gas temperature of about 200° C. should not be exceeded, since at higher temperatures the requirements and hence the costs of the conveying means rise to a great extent.
Therefore, it is the object of the present invention to improve the process to the effect that the temperature of the gas at the inlet of the condenser or the blower of the fluidized-bed reactor can be adjusted such that both urea deposits and an excessive thermal stress of the conveying means is avoided.